Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. The formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer, and thus, the porous layer forms a reservoir in which hydrocarbons are able to collect. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In what is perhaps the most basic form of rotary drilling methods, a drill bit is attached to a series of pipe sections referred to as a drill string. The drill string is suspended from a derrick and rotated by a motor in the derrick. A drilling fluid or “mud” is pumped down the drill string, through the bit, and into the well bore. This fluid serves to lubricate the bit and carry cuttings from the drilling process back to the surface. As the drilling progresses downward, the drill string is extended by adding more pipe sections.
When the drill bit has reached the desired depth, larger diameter pipes, or casings, are placed in the well and cemented in place to prevent the sides of the borehole from caving in. Once the casing is cemented in place, it is perforated at the level of the oil bearing formation so hydrocarbons can enter the cased well. If necessary, various completion processes are performed to enhance the ultimate flow of hydrocarbons from the formation. The drill string is withdrawn and replaced with production tubing. Valves and other production equipment are installed in the well so that the hydrocarbons may flow in a controlled manner from the formation, into the cased well bore, and through the production tube up to the surface for storage or transport.
That simplified example of an oil and gas well, comprising as it does a single casing and a single tube, is not often encountered in the real world. Given the depth of most producing oil and gas wells and various environmental considerations, they more commonly incorporate a number of pipes or “tubulars” of varying diameters. Casings of diminishing diameter may be “telescoped” together to extend the depth of the well. Multiple casings also may be nested in each other. For example, the upper portion, that is the wellhead or “tree” of a subsea well usually will comprise a number of nested, or coextending tubulars.
Very typically, a subsea tree will include a very large casing, what is called a conductor, with a diameter of 30 or more inches which is cemented in the well. A somewhat smaller diameter, but typically longer “surface” casing is nested in the conductor and cemented in place. The tree may include a smaller “intermediate” casing, but usually will include an even smaller “production” casing, which extends beyond its surrounding casings down to a hydrocarbon bearing formation. Finally, production tubing will be suspended inside the production casing.
While such complex well designs allow hydrocarbons to be produced safely and efficiently from even very deep subsea wells, they present significant challenges in “plug and abandonment, so-called “P&A” jobs. That is, eventually a well may be depleted to the point where further production is no longer economical. At that point, the well must be decommissioned by, inter alia, cutting the subsea tree at a minimum depth of a least 20 feet and then plugging the well.
Without minimizing the complexity of the overall P&A job, cutting the tree can present significant challenges. Production tubing, since it typically is suspended inside production casing, may be removed relatively easily by pulling it from the well. The various casings nested inside each other, however, usually are cemented in whole or in part and cannot be pulled. They must be cut in the well. The task is further complicated by the fact that the various casings often are not situated concentrically to each other, but often are displaced relative to the conductor axis and are eccentrically nested.
Most commonly, and certainly most preferably, the casing is cut from the inside out by attaching a cutting tool to the end of a work string and running it down into the casing. Typically such tools incorporate a set of three identical blades, although smaller diameter tools may incorporate only two blades. The blades are pivotally mounted to the body of the tool in a common plane, and are disposed symmetrically about the tool's primary axis. When the tool is being run into the casing the blades are in a closed or retracted position nesting in the body of the tool. Once at the desired depth, the tool is rotated via the work string. The blades are actuated and pivot outward, cutting the casing in the process.
Examples of such cutting tools include those disclosed in U.S. Pat. No. 7,909,100 to C. Bryant, Jr. et al. and U.S. Pat. No. 7,063,155 to D. Ruttley. Other examples include cutting tools commercially available from Pioneer Oil Tools Limited (Scotland), Drillstar Industries (France), and the Servco division of Schlumberger Limited (France), such as those disclosed in the following marketing materials: Type “CCH” Hydraulic Casing Cutter, Pioneer Oil Tools Limited (2010); Hydraulic Casing Cutter, Drillstar Industries; Extended Reach Hydraulic Pipe Cutter, Servco (2011); and Hydraulic Pipe Cutter, Servco (2011). Those cutting tools incorporate various mechanisms for actuating the cutting blades and for determining when the blades have been fully extended and the full extent of their cut diameter has been reached. They also may be provided with blades of different lengths to provide the tool with a greater or lesser cut diameter. They all, however, incorporate a single set of pivoting cutting blades, all of the same length.
If the wellhead includes a number of casings, such as the tree of a deep subsea well, the process of cutting all the casings and pulling them from the well often must proceed in stages. That is, a first cutting tool having a relatively smaller cutting diameter is lowered into the innermost casing. The casing is cut, the tool is retrieved, and the cut casing then is pulled from the well. Another cutting tool, having a larger cut diameter, then is run into the remaining casing and the process repeated. If necessary, the process is repeated with yet another cutting tool having an even larger cut diameter until all casing in the well has been cut and pulled.
Multiple cutting trips most commonly are required in shutting down deep, subsea well, but ironically, those are the situations where multiple trips are the most costly. Apart from capital expenses for equipment, operating costs for modern offshore rigs can be $500,000 or more a day. Ever increasing operational costs of drilling rigs makes it increasingly important to combine operations so as to reduce the number of trips into and out of a well and to reduce the time spent by a rig on site.
Accordingly, there remains a need for new and improved systems, apparatus and methods for cutting casings in oil and gas wells. Such disadvantages and others inherent in the prior art are addressed by various aspects and embodiments of the subject invention.